SU‐E‐T‐417: The Impact of Normal Tissue Constraints On PTV Dose Homogeneity for Intensity Modulated Radiotherapy (IMRT), Volume Modulated Arc Therapy (VMAT) and Tomotherapy

Abstract

Purpose:Complex intensity modulated arc therapy tends to spread low dose to normal tissue(NT)regions to obtain improved target conformity and homogeneity and OAR sparing. This work evaluates the trade‐offs between PTV homogeneity and reduction of the maximum dose(Dmax)spread to NT while planning of IMRT,VMAT and Tomotherapy.Methods:Ten prostate patients,previously planned with step‐and‐shoot IMRT,were selected. To fairly evaluate how PTV homogeneity was affected by NT Dmax constraints,original IMRT DVH objectives for PTV and OARs(femoral heads,and rectal and bladder wall)applied to 2 VMAT plans in Pinnacle(V9.0), and Tomotherapy(V4.2). The only constraint difference was the NT which was defined as body contours excluding targets,OARs and dose rings.NT Dmax constraint for 1st VMAT was set to the prescription dose(Dp).For 2nd VMAT(VMAT_NT)and Tomotherapy,it was set to the Dmax achieved in IMRT(~70‐80% of Dp).All NT constraints were set to the lowest priority. Three common homogeneity indices(HI),RTOG_HI=Dmax/Dp,moderated_HI=D95%/D5% and complex_HI=(D2%‐D98%)/Dp*100 were calculated.Results:All modalities with similar dosimetric endpoints for PTV and OARs. The complex_HI shows the most variability of indices,with average values of 5.9,4.9,9.3 and 6.1 for IMRT,VMAT,VMAT_NT and Tomotherapy,respectively.VMAT provided the best PTV homogeneity without compromising any OAR/NT sparing.Both VMAT_NT and Tomotherapy,planned with more restrictive NT constraints,showed reduced homogeneity,with VMAT_NT showing the worst homogeneity(P<0.0001)for all HI. Tomotherapy gave the lowest NT Dmax,with slightly decreased homogeneity compared to VMAT. Finally, there was no significant difference in NT Dmax or Dmean between VMAT and VMAT_NT.Conclusion:PTV HI is highly dependent on permitted NT constraints. Results demonstrated that VMAT_NT with more restrictive NT constraints does not reduce Dmax NT,but significantly receives higher Dmax and worse target homogeneity. Therefore, it is critical that planners do not use too restrictive NT constraints during VMAT optimization. Tomotherapy plan was not as sensitive to NT constraints,however,care shall be taken to ensure NT is not pushed too hard. These results are relevant for clinical practice. The biological effect of higher Dmax and increased target heterogeneity needs further study.